Open-pit sunken planting: a tree establishment technique for dry environments

Abstract

The commonly practiced ground-level planting was compared with a moisture-conserving open-pit sunken planting technique in an attempt to test the success of modified planting techniques that enhance tree survival and early growth in arid environments. An indigenous species, Acacia mellifera (Vahl.) Benth., which is well adapted to the environmental conditions of the trial site, was used for the comparative study. Trial results showed that, after 21 months, height growth of seedlings planted in open pits was almost three times faster than that of seedlings planted at ground level. Total biomass production of open pit planted seedlings exceeded that of those planted at ground level by 78%. Shoot ÷ root ratios of biomass produced were 1 ÷ 1.07 in sunken plantings and 1 ÷ 1.43 in level planting, suggesting that roots of sunken planted seedlings support a higher above-ground biomass production. No mortalities occured in either of the two treatments because of the natural drought tolerance of A. mellifera. The highly significant differences in height growth performance suggested that this planting technique should be tested on other tree species on a larger scale. Accordingly, Leucaena leucocephala (Lam.) de Wit was used for a second, larger experiment. The results of this trial largely confirmed those of the preceding one. After 31 months, the increase in shoot length of seedlings planted in open pits was 50% higher than that of seedlings planted at ground level. However, total biomass production was only 13% higher for seedlings planted in open pits and no significant differences between the two treatments could be established. Nevertheless, there was a significant difference between the species' shoot-to-root ratios. This ratio was 1 ÷ 1.04, for the sunken planting treatment, compared to 1 ÷ 1.60 for level planting. Mortality among seedlings in the open pits was half that of the seedlings planted at ground level.